Mercruiser 5.0 Carb'd - Bogging

[20chap04]

Looking for some advice for trouble shooting my 2004 5.0 Mercruiser....Idles fine, but won't accelerate...try to coax it by working the throttle and it boggs and "pops" (no backfire yet). When I try to accelerate, I can hear the carb jets making a sucking sound...

Would like to check for the "simple" stuff before sending it to a mechanic.

Any thoughts or suggestions would be appreciated.

[Fubar512]

Without being there to diagnose the problem, and going by what you've described, I'd say it's a carburetor or a timing issue....either spark or valve timing. On a SB Chevy, the later usually means a worn timing chain that's jumped a few teeth on the cam gear (sprocket). My reasoning? The popping and excessive intake roar.

Lets look at the simple things, first: Have you changed the fuel filters? Checked the fuel pump? Is the carb getting enough fuel? By chance, does that engine have a distributor, and if so, have you ever pulled it for any reason?

[20chap04]

Fubar, thanks for your reply.

Will be checking the fuel filter and water seperator tonight, those were a couple of things that were on my short list as well. It does have a distributor but it has never been removed. I've owned this boat since new. It's got 220 hours on it so I'm assuming that a worn timing chain/gear would be unlikely.

I've got to admit, other than standard maintenance, (oil & filter, lube changes, spark plugs) I haven't had anything done in the way of a tune up. On the other hand, it's run like a top since new so I was in the " if its not broken, don't fix it" camp. It has always been winterized and stored with adequate fuel stabilizer, I did store it with very little fuel this winter since I couldn't find any ethanol free fuel in my area.

[Fubar512]

Then that effectively narrows it down to carburetion and fuel supply.

Is the boat used in salt water? I know it's a long shot, but I've experienced a situation where a boat has sucked salt spray in through it's tank vent. During the course of three seasons, that particular boat had ingested several gallons of seawater, which overwhelmed the water separator, and subsequently did a number on the carburetor. When we took that carb apart, we found the float bowls badly pitted.

Another possibility, of course, is ethanol-blended fuel. Look for a chalky, powder-like residue in the filter(s). That's oxidization being "cleaned" off your fuel tank walls by the ethanol.

[20chap04]

Freshwater only use, but you do have me wondering about possible blockages in the vent line (nesting insects etc.)

Up until Saturday, I've never used an ethanol blend in the boat....used to get the ethanol free stuff in the surrounding counties where I live...now it seems impossible to find.

I'm too am beginning to believe it's a fuel related issue, boat was running perfectly late last year so in my mind, that's a likely culprit. It just seems to be starving for fuel. I'll keep you posted on how things turn out.

Again, thank you for your input.

[TooReel]

Check your accelerator pump. With the engine off move the throttle linkage and make sure it is getting a good squirt of gas.

[20chap04]

TooReel & Fubar,

Checked the accelerator pumps and they seem to be working fine. Strong squirt of fuel when advancing the throttle.

Fuel fillter was perfectly clear, no water in the fuel seperator and no apparent vent blockages.

However.....when I pulled the distributor cap there was a lot of corrosion on the rotor contact points and on the contacts for the individual spark plug location.

Should I clean or replace the cap? I'll replace the rotor....does it pull straight off or is it retained somehow?? What else should I do to get this straight?? Unfortunately, I don't have a manual and its been a bunch of years since I've fooled around with this type of setup.

Thanks again to both of you for your advice.

[Fubar512]

Yes, replace both the cap and rotor. They should always be replaced as a set (to maintain the gap between the rotor tip and the cap electrodes).

Some rotors are secured in place with a "tang" that requires no more than a squeeze along the shaft housing while you simultaneously lift it out. Older Mercruisers and Crusaders pretty much used a standard-issue Delco distributor, and the rotor was held down with two screws through the spark-advance plate (just as it was on an old Chevy).

[midway]

While in the distributor check the mechanical advance in it for broken or missing springs, stiff movement, since you have corrosion on the cap and rotor there may be more.

[Sandy]

Definitely replace. And always check/reset the timing (in timing mode)when replacing (or even just cleaning) cap & rotor. Suggest vac'ing out any corrosion particles that may have fallen down to the plate or below as well. And put a few drops of light oil on the shaft bearing wick under the rotor.

Midway- Good suggestion, but would a 2004 still have mechanical advance?, ..dunno.